Many different standards and modulation schemes exist, but one of the most prevalently used in the world of mobile terminals is the Global System for Mobile Communications (GSM). One of the modulation schemes of the GSM standard is the Enhanced Data Rates for GSM Evolution (EDGE) modulation scheme. The EDGE modulation scheme contains an amplitude modulation (AM) component and a phase modulation (PM) component. Since there is an amplitude modulation component, the power amplifier of a transmitter operating according to the EDGE modulation scheme must be linear or driven according to a polar modulation scheme.
If a polar modulation scheme is used, a phase modulated signal at the desired radio frequency is provided to the input of the power amplifier and an amplitude modulation component is used to vary the supply voltage provided to the power amplifier. As a result, the power amplifier may operate in saturation and efficiency is greatly improved. Unfortunately, the amplitude modulation component that controls the supply voltage provided to the power amplifier causes unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called Amplitude Modulation to Phase Modulation (AM/PM) distortion, and it degrades the spectral purity of the system and the Error Vector Magnitude (EVM). An “AM/PM curve” describes the phase relationship between the AM component and the PM component over a range of power control voltages.
If the amplifier is used in a polar modulated application, it becomes critical to maintain consistent and repeatable amplitude and phase versus the power control voltage in the power amplifier. This is due to the accuracy in the amplitude and phase paths required to reproduce a signal that is spectrally clean enough to pass international standards. In an ideal power amplifier, the AM/PM curve is only dependent on the supply voltage set by the collector regulator and is independent of varying conditions such as battery supply voltage, input power, and temperature. However, the power amplifier often exhibits variations that are dependent on temperature.
Specifically, the base-emitter voltage drop Vbe of a GaAs Heterojunction Bipolar Transistor (HBT) will decrease by roughly 2 mV per degree Celsius. When GaAs HBTs are used in a power amplifier, if the base bias voltage Vbias is held fixed as temperature varies, the resulting shift in base-emitter voltage drop Vbe will cause a change in quiescent current, which changes the quiescent operating condition, hereinafter referred to as the “operating condition.” The operating condition influences the relative level of compression of each stage in the amplifier, which in turn directly affects the shape of the AM/PM curve.
Thus, there is a need to provide the power amplifier with a temperature-compensated bias voltage VbiasTC that is independent of supply voltage and dependent upon temperature, such that as the operating temperature of the power amplifier varies, the temperature-compensated bias voltage VbiasTC is adjusted appropriately so as to maintain a consistent operating condition in the power amplifier. Such temperature compensation provides a more consistent reproduction of the phase versus control voltage profile and allows better output RF spectrum (ORFS) performance over temperature.